1. Field of the Invention
The present invention relates to a multilayer polypropylene film of high shrinkage which comprises at least one base layer containing a propylene polymer or a propylene polymer mixture and at least one outer layer which comprises a mixture or a blend of two polymer components.
The film according to the present invention is distinguished by a characteristic matte surface or appearance and high shrinkage in the longitudinal and transverse directions.
The invention further relates to a process for the production of the multilayer film and to the use of the film.
2. Description of Related Art
Shrink films are known and are widely used as packaging films.
Shrink packaging is usually produced starting from a pouch, into which the goods to be packaged are introduced, followed by separation/welding and subsequent shrinkage in a shrink oven. In this manner, very tight, fold-free film packaging is obtained. In order for this tight wrap to be reliably achieved, the films used must have high shrinkage values, since the film wrapping after separation/welding surrounds the contents relatively loosely. This looseness is due to the fact that the welding wire cannot be moved right up against the packaged goods. Very high shrinkage values of the films are therefore a prerequisite for their use as shrink packaging. Such films are described, for example, in EP-A-0 436 196, DE-A-19 54 467, DE-A-32 15 120 and EP-A-0 350 859.
Conventional shrink films are generally distinguished by very high sheen values and very low haze. For certain areas of application of shrink films, this appearance is undesirable, even disadvantageous. For example, the legibility of an imprint on packaged goods can be impaired due to reflection at the glossy surface of the packaging film.
In addition, all known shrink films are unsatisfactory with respect to their passage through machines, their antistatic properties, their properties during winding and their properties during further processing and slitting.
EP-A-0 489 373 discloses heat-sealable shrink films built up from a polyropylene (PP) homopolymer base layer and ethylene-propylene copolymer outer layers and which have high sheen and low haze. These films have increased shrinkage compared with other conventional heat-sealable packaging films, but the shrink properties are inadequate if tight coverings are required.
German Patent Application P 41 35 096.0 discloses multilayer polyolefin films comprising a polypropylene base layer and an outer layer made from an HDPE blend. This HDPE blend comprises HDPE and copolymers and/or terpolymers made from .alpha.-olefins and, if desired, polypropylene. The outer layer of the multilayer film has minimum sheen and maximum haze, giving the film a characteristic matte appearance.
U.S. Pat. No. 4,578,316 discloses a biaxially oriented multilayer polyolefin film whose base layer comprises a polypropylene homopolymer and whose outer layer comprises a blend of polypropylene and MDPE and/or HDPE. This polyolefin film has a low coefficient of friction together with a readily wettable surface. However, this film also has excellent optical transparency.
WO 89/10839 describes multilayer polyolefin films whose base layer comprises a polypropylene polymer and whose outer layers comprise either HDPE or an ethylene-propylene copolymer or polypropylene. The film is stretched under particular temperature conditions. It has a low coefficient of friction and good wettability, but simultaneously has high sheen and low haze.
EP-A-0 367 613 discloses a multilayer film comprising a vacuole-containing polypropylene base layer and an inscribable outer layer which contains a first polymer having a melt flow index of .ltoreq.1 g/10 min and a second polymer which is incompatible with said first polymer. Incompatible meaning that the polymers are present as separate phases. An example given of the first polymer is an HDPE having a density of 0.92 to 0.97 g/cm.sup.3. The incompatible polymer is polypropylene or a copolymer of propylene or a terpolymer of propylene. The film disclosed has an opaque appearance, i.e. it is essentially non-transparent to light. The film described has a density of 0.69 g/cm.sup.3. This film cannot be employed for many applications due to its opacity.